There is a need in the art for polymeric blends having a stereospecific polypropylene component with good tensile strength while still providing suitable mechanical recoverability (elastic recovery) and flexibility (low flexural modulus). This invention is aimed at improving the aforementioned properties of blends having a stereoregular polypropylene component, especially isotactic polypropylene. This is achieved by blending the stereoregular polypropylene component with a copolymer of propylene and a C2, C3-C20 α-olefin. This copolymer is less crystalline than the isotactic polypropylene. In the copolymer the propylene is polymerized substantially stereospecifically. Most preferably, the copolymer is an ethylene propylene copolymer, e.g., ethylene propylene thermoplastic elastomer. The copolymer has a substantially uniform composition distribution preferably as a result of polymerization with a metallocene catalyst Composition distribution is a property of copolymers indicating a statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later.
Blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art. However, the traditional Ziegler-Natta catalysts cannot make ethylene propylene thermoplastic elastomers which simultaneously are uniform in compositional distribution, have substantially stereospecific propylene residues and have less than 35 wt. % ethylene.
U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers. In U.S. Pat. No. 3,888,949 Chi-Kai Shih, assigned to E I Du Pont, shows the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term “block” in the description of the polymer where the copolymer is described as having “sequences of different alpha-olefin content.” Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
In U.S. Pat. No. 4,461,872, A. C. L. Su improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system which is expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
In two successive publications in the journal of Macromolecules, 1989, V22, pages 3851-3866, J. W. Collette of E. I. Du Pont has described blends of isotactic polypropylene and partially atactic polypropylene which have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described by R. Waymouth in U.S. Pat. No. 5,594,080, in the article in the Journal American Chemical Society (1995), Vol. 117, page 11586, and in the article in the Journal American Chemical Society (1997), Vol. 119, page 3635, J. Chien in the journal article in the Journal of the American Chemical Society (1991), Vol. 113, pages 8569-8570; and S. Collins in the journal article in Macromolecules (1995) Vol. 28, pages 3771-3778. These articles describe a specific polymer, but do not describe the blends with a more crystalline polymer such as isotactic polypropylene.
In U.S. Pat. Nos. 3,853,969 and 3,378,606, E. G. Kontos discloses the formation of in situ blends of isotactic polypropylene and “stereo block” copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the “stereo block” character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend. Moreover, all of these compositions either do not meet all of the desired properties for various applications, and/or involve costly and burdensome process steps to achieve the desired results.
Similar results are anticipated by R. Holzer and K. Mehnert in U.S. Pat. No. 3,262,992 assigned to Hercules wherein the authors disclose that the addition of a stereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. These copolymers were synthesized by changing the monomer concentrations in the reactor with time. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60). The presence of stereoblock character in these polymers is shown by the high melting point of these polymers and the poor solubility in hydrocarbons at ambient temperature.
There is a need for a polyolefin blend composition which is thermally stable, heat resistant, light resistant and generally suitable for thermoplastic elastomer (TPE) applications which has advantageous processing characteristics. We have found that by blending a crystalline propylene polymer, hereinafter referred to as the “first polymer component, (FPC)” and a crystallizable propylene alpha olefin copolymer polymer, hereinafter referred to as the “second polymer component (SPC)”, advantageous processing characteristics result while still providing a composition having decreased flexural modulus and increased tensile strength, elongation, recovery and overall toughness. It is possible to have the addition of a third polymeric component which is another crystallizable propylene alpha olefin copolymer indicated as SPC2 in the text below which has crystallinity intermediate between the FPC and the SPC. The SPC2 also has a narrow composition distribution and is made with a metallocene catalyst. The addition of SPC2 leads to a finer morphology and improvements in some of the properties of the blend of FPC and SPC.
The term “crystalline,” as used herein for FPC, characterizes those polymers which possess high degrees of inter- and intra-molecular order, and which melt higher than 110° C and preferably higher than 115° C. and more preferably higher than 130° C. and preferably have a heat of fusion of at least 75 J/g, as determined by DSC analysis. And, the term “crystallizable,” as used herein for SPC describes polymers which are mainly amorphous in the undeformed state, but can crystalize upon stretching or annealing. Crystallization may also be initiated by the presence of a crystalline polymer such as the FPC. These polymers have a melting point of less than 105° C. or preferably less than 100° C. and preferably have a heat of fusion of less than 75 J/g as determined by DSC. SPC2 describes those polymers that are substantially crystalline in the undeformed state. Further crystallization may also occur in the presence of the crystalline polymer such as FPC. These polymers have a melting point of less than 115° C. or preferably less than 100° C. and preferably have a heat of fusion of less than 75 J/g as determined by DSC.